Mix the remaining 3 eggs one at a time, beating well after each addition. Combine flour, cocoa, baking powder, and salt; mix into the batter. Stir in chocolate chips. Remove 1 C. of the chocolate batter. Spread the remaining batter into the prepared pan. Spread the peanut butter filling over the top. Drop the reserved chocolate batter by teaspoonful over the filling. Using a knife, gently swirl through the top layers for a marbled affect. Bake in preheated oven for 35-40 minutes, or until a wooden toothpick inserted near the center comes out almost clean. Cool completely, then cut into bars.